PLASTIC ENCAPSULATED INTEGRATED CIRCUIT PACKAGE WITH FULLY SLOTTED DAMBAR

Abstract: A plastic encapsulated integrated circuit package is disclosed herein. This IC package (24) includes a lead frame (12) having an IC attachment pad, a lead chip, or other suitable means (14) for supporting an IC chip, a plastic molding encapsulating (26) the IC pad and chip (16), and spaced-apart but adjacent electrically conductive leads (28) forming part of the lead frame and extending out from the plastic molding (26). The lead frame also includes an electrically conductive dambar (30) extending across the leads (28) at points adjacent to and just outside the plastic molding (26). This particular dambar (30), as disclosed, includes initially formed through-slots (32) between adjacent ones of the leads (28), that is, through-slots (32) that are provided before the package is encapsulated with plastic. This through-slotted dambar (30) is especially designed for use with a fine pitched IC package and functions in the normal plastic damming manner, notwithstanding its through-slots (32) which are sufficiently large to receive the following plastic as the package is encapsulated but small enough to prevent the plastic from flowing beyond the dambar (30).

Description: PLASTIC ENCAPSULATED INTEGRATED CIRCUIT PACKAGE WITH FULLY SLOTTED DAMBAR The present invention relates generally to plastic encapsulated integrated circuit packages, and more particularly, to a specifically designed through-slotted dambar especially suitable for use in encapsulating fine pitched integrated circuit package. An ultimately formed plastic encapsulated integrated circuit package of the type to which the present invention is directed is illustrated in Figure 1 and generally designated by the reference numeral 10. This IC package is shown including a lead frame 12 having an IC attachment pad, lead tip, or other suitable means 14 for supporting an IC chip 16. A plastic molding 18 which is formed in the manner to be described immediately below encapsulates the IC pad and IC chip in the manner shown. Spaced-apart but adjacent electrically conductive leads 20 forming part of the lead frame extend out from the plastic molding, on all sides of the latter in the particular embodiment depicted in Figure 1. Once the IC package, absent its plastic encapsulation, is assembled in the normal manner, it is positioned within cooperating encapsulation molds (not shown), after which, viscous plastic is injected into the molds and thereafter cooled to form a two piece plastic molding 18. During this process, there is a tendency for the viscous plastic to be injected out of the molds by flowing between adjacent leads 20 which, by design, extend beyond the molds. In order to limit this outward movement of flowing plastic during the molding process, the typical prior art packaging technique utilizes a dambar outside but in close proximity to the molds. One such prior art dambar is illustrated in Figure 2 which, as will be seen, is an enlarged view of a portion of the IC package shown in Figure 1. This particular dambar 22, which is electrically conductive, is shown extending across and integrally formed with outwardly protruding leads 20 in parallel relationship with and just beyond one edge of molding 18. The plastic material that escapes the molds during the encapsulating process is shown at 23 filling the spaces between leads 20 inside the dambar only, which is the purpose of the dambar. Dambar 22 has been described as an electrically conductive integral part of leads 20. As a result, in order to electrically isolate the leads from one another, the dambar eventually must be removed, that is, after molding 18 has been formed. As a general rule, this process is difficult and expensive, although this particular approach has been used widely for almost all semiconductor package assemblies since the very beginning. More recently however, IC packaging designers have resorted to what may be referred to as fine pitched high density IC packages, that is, packages where the pitch of the leads is equal to or less than 20 mils. In these cases, the removal of the dambar becomes extremely difficult and increasingly more expensive, especially as the designers contemplate finer and finer pitched IC packages. As will be seen hereinafter, the present invention provides an uncomplicated and yet reliable solution to the problem described immediately above. In particular, as will be seen, the present invention, like the prior art, contemplates the use of an electrically conductive dambar extending across the outwardly protruding leads at points adjacent to and just outside the plastic molding. However, in accordance with the present invention, the dambar is initially provided with through-slots between adjacent ones of the leads. These through-slots are made sufficiently large to receive viscous plastic flowing out of the molds between the leads during the encapsulation process, but sufficiently small to contain most if not substantially all of from the flowing plastic, preventing it from excessively passing beyond the dambar. Thus, this through-slotted dambar serves the same damming function as dambar 22 described above while, at the same time, the plastic filled through-slots electrically isolate the leads from one another and thereby eliminate the necessity to remove the dambar. The present invention will be described in more detail hereinafter in conjunction with the drawings, wherein: FIGURE 1 is a partially broken away perspective view of a plastic encapsulated integrated circuit package formed in accordance with the prior art; FIGURE 2 is an enlarged plan view of a portion of the package shown in Figure 1 , specifically illustrating a typical dambar forming part of the overall package; FIGURE 3 is a plan view similar to Figure 2 but illustrating a through-slotted dambar designed in accordance with the present invention; and FIGURE 4 is an enlarged diagrammatic illustration of a portion of the dambar of Figure 3, specifically depicting one of its through-slots. Turning to the drawing, attention is immediately directed to Figure 3 where one edge of a plastic encapsulated integrated circuit package designed in accordance with the present invention is shown in enlarged plan view. This IC package, which is generally indicated by the reference numeral 24, includes a lead frame having an IC attachment pad, lead tip, or other suitable means for supporting an IC chip in the same manner as IC package 10, as well as a plastic molding encapsulating the IC pad and chip. The plastic molding is shown at 26 in Figure 3. As also seen in this figure, the package's lead frame includes spaced-apart but adjacent electrically conductive leads 28 and an electrically conductive dambar 30 extending across the leads at points adjacent to and just outside plastic molding 26. Dambar 30 is actually made up of a series of cross members integrally formed with respective leads 28 so as to define through-slots 32 between adjacent ones of the leads. One such through-slot 32, illustrated in Figure 4, has been enlarged for purposes of clarity. Note specifically that the through-slot is generally somewhat longer than it is wide, although this is not necessarily required and it is narrower than the normal spacing between adjacent electrically conductive leads 28. The width and length of each slot depends in large part on the type of plastic making up molding 18, its viscosity during the encapsulating process, and the degree to which the viscous plastic flows out of the molds and into the through slots 32. The length and width of these slots are specifically designed to receive but then contain most if not substantially all of the flow of plastic, thereby preventing most or substantially all of the viscous material from passing beyond dambar 30, as illustrated in Figure 3. In this way, the dambar serves its usual function of preventing further movement of the viscous plastic, in the same manner as previously described dambar 22. However, at the same time, the plastic filled through-slots serve to electrically insulate leads 28 from one another. Hence, it is not necessary to remove the dambar, as in the case of dambar 22, illustrated in Figure 2. In a specific embodiment of the present invention, the encapsulating plastic is an off-the shelf epoxy. For an IC package having a fine pitch, 20 mils or less, each of the through-slots 32 is preferably between 3 and 4 mils wide and between 10 and 15 mils long. Obviously, these dimensions will vary depending upon the pitch of the overall IC package and the particular encapsulating plastic that is used and other post- mold processes. One with ordinary skill in the art to which the present invention is directed, can readily provide the appropriately sized through-slots in view of the teachings herein. It is important to note that through-slotted dambars could be incorporated into course-pitched IC packages. However, the courser the IC package is, the more costly it is to provide through slots. In a preferred embodiment, these through-slots are formed by means of either stamping or etching.

Claims: WHAT IS CLAIMED IS: 1. In a plastic encapsulated integrated circuit package including a lead frame having means which supports an IC chip, a plastic molding encapsulating said IC support means and chip, and spaced-apart but adjacent electrically conductive leads forming part of said lead frame and extending out from said plastic molding, the improvement comprising an electrically conductive dambar extending across said leads at points adjacent to and just outside said plastic molding, said dambar including through-slots between adjacent ones of said leads before said IC support means and chip are encapsulated in said plastic molding, said slots being filled with the same plastic as said plastic molding whereby to electrically isolate the leads from one another. 2. The improvement according to Claim 1 wherein said dambar is integrally formed with said electrically conductive leads so as to initially includes said through-slots. 3. The improvement according to Claim 1 wherein said electrically conductive leads are spaced apart from center point to center point at most about 20 mils. 4. The improvement according to Claim 3 wherein the width of each of said dambar slots is between about 3 and 4 mils and the length of each of said slots is between about 10 and 15 mils. 5. An integrated circuit package, comprising: (a) a lead frame including means adapted to support an IC chip, spaced apart but adjacent electrically conductive leads extending out from said IC support means, and an electrically conductive dambar extending across said leads at points adjacent to but spaced just outside said IC support means, said dambar including through-slots between adjacent ones of said leads; (b) an IC chip supported on said IC support means inward of said through-slotted dambar; (c) a plastic molding encapsulating said IC support means and IC chip inward of said through-slotted dambar; and (d) plastic molding material of the same type making up said plastic molding being disposed within the through-slots of said dambar, whereby to electrically isolate the electrically conductive leads from one another. 6. An integrated circuit package according to Claim 5 wherein said dambar is initially integrally formed with said electrically conductive leads so as to initially include said through-slots. 7. An integrated circuit package according to Claim 6 wherein said electrically conductive leads are spaced apart from center point to center point at most about 20 mils. 8. An integrated circuit package according to Claim 7 wherein the width of said dambar slots is about 3 and 4 mils, and the length of said slots is about 10 and 15 mils. 9. A lead frame for use as part of a plastic encapsulated integrated circuit package, said lead frame comprising: (a) means adapted to support an IC chip and adapted to be encapsulated by a plastic molding along with said chip; (b) spaced-apart but adjacent electrically conductive leads extending out from said IC support means beyond said plastic molding when the latter is provided; and (c) an electrically conductive dambar extending across said leads at points adjacent to and just outside of said plastic molding when the latter is provided, said dambar including through-slots between adjacent ones of said leads. 10. A lead frame according to Claim 9 wherein said dambar is integrally formed with said electrically conductive leads so as to initially include its through-slots. 11. A lead frame according to Claim 10 wherein said electrically conductive leads are spaced apart from center point to center point at most about 20 mils. 12. A lead frame according to Claim 11 wherein the width of said dambar slots is between about 3 and 4 mils and the length of said slots is between about 10 and 15 mils. 13. A method of forming a plastic encapsulated integrated circuit package, comprising the steps of: (a) providing a lead frame having means adapted to support an IC chip, spaced apart but adjacent electrically conductive leads extending out from said IC support means, and an electrically conductive dambar extending across said leads at points adjacent to and just outside said IC support means, said dambar including through-slots between adjacent ones of said leads; (b) positioning an IC chip on said IC support means; and (c) encapsulating said IC pad and chip with plastic inwardly of said through-slotted dambar. 14. A method of forming a plastic encapsulated integrated circuit package according to Claim 13 wherein said IC support means and IC chip are encapsulated with said plastic in a way which causes said plastic to flow into the through-slots of said dambar in order to electrically isolate said leads from one another. 15. A method of forming a plastic encapsulated integrated circuit package according to Claim 14 wherein said electrically conductive leads are spaced apart from center point to center point about 20 mils. 16. A method of forming a plastic encapsulated integrated circuit package according to Claim 15 wherein the width of said dambar slots is between about 3 and 4 mils and the length of said slots is between about 10 and 15 mils.

Inventors: OOI CHRIS B K, OOI, CHRIS, B., K.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1994
Main classification: H01L23
IPC codes: H01L23